The present invention relates to controlled energization of electric motors, more particularly to applying power on a dynamic basis to an optimum number of coils wound on stator poles for maximum efficiency for each of a plurality of operating speed ranges.
The progressive improvement of electronic systems, such as microcontroller and microprocessor based applications for the control of motors, as well as the availability of improved portable power sources, has made the development of efficient electric motor drives for vehicles, as a viable alternative to combustion engines, a compelling challenge. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of a battery source to appropriate stator windings, functional versatility that is virtually indistinguishable from alternating current synchronous motor operation can be achieved.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics. In a vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. The copending related U.S. application incorporates electromagnet poles as isolated magnetically permeable structures configured in an annular ring, relatively thin in the radial direction, to provide advantageous effects. With this arrangement, flux can be concentrated, with virtually no loss or deleterious transformer interference effects in the electromagnet cores, as compared with prior art embodiments.
The Maslov et al. applications recognize that isolation of the electromagnet groups permits individual concentration of flux in the magnetic cores of the groups, with virtually no flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an autonomous electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched.
The above-identified copending U.S. patent application Ser. No. 10/173,610 is directed to a control system for a multiphase motor having these structural features. A control strategy is described that compensates for individual phase circuit characteristics and offers a higher degree of precision controllability since each phase control loop is closely matched with its corresponding winding and structure. Control parameters are specifically matched with characteristics of each respective stator phase. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with the parameters associated with the stator phase component for the phase winding energized.
While the motors described in the above-identified applications provide operational advantages, these motors and prior art motors do not exhibit uniformly high efficiency at all speeds of a wide operating speed range, even with non-variable loads. For a fixed motor topology, the available magnetomotive force (MMF) is dependent upon the number of winding turns and energization current. The term xe2x80x9cmotor topologyxe2x80x9d is used herein to refer to physical motor characteristics, such as dimensions and magnetic properties of stator cores, the number of coils of stator windings and wire diameter, etc. The available magnetomotive force dictates a variable, generally inverse, relationship between torque and speed over an operating range. An applied energization current may drive the motor to a nominal operating speed. As the motor accelerates toward that speed, the torque decreases, the current drawn to drive the motor decreases accordingly, and thus efficiency increases to a maximum level. As speed increases beyond the level of peak efficiency, additional driving current is required, thereby sacrificing efficiency thereafter. Thus, efficiency is variable throughout the speed range and approaches a peak at a speed well below maximum speed.
Motors with different topologies obtain peak efficiencies at different speeds, as illustrated in FIG. 1. This figure is a plot of motor efficiency versus operating speed over a wide speed range for motors having different topologies. The topologies differ solely in the number of stator winding turns. Each efficiency curve approaches a peak value as the speed increases from zero to a particular speed and then decreases toward zero efficiency, the curve being generally symmetrical. Curve A, which represents the greatest number of winding turns, exhibits the steepest slope to reach peak efficiency at the earliest speed V2. Beyond this speed, however, the curve exhibits a similarly steep negative slope. Thus, the operating range for this motor is limited. The speed range window at which this motor operates at or above an acceptable level of efficiency, indicated as X% in the figure is relatively narrow. Curves B-E successively represent with fewer winding turns. As the number of winding turns decreases, the slope of each efficiency curve decreases and the speed for maximum efficiency increases. Curve B attains peak efficiency at speed V3, curve C at V4, curve D at V5 and curve E at V6. As the negative slope of each curve beyond the peak efficiency speed is of similar configuration to the initial slope, the acceptable efficiency speed range window increases.
In motor applications in which the motor is to be driven over a wide speed range, such as in a vehicle drive environment, FIG. 1 indicates that there is no ideal single motor topology that will provide uniformly high operating efficiency over the entire speed range. For example, if the maximum operating speed is to be V6 or greater, motor topologies for curves A and B indicate zero efficiency at maximum speed, while curve C exhibits significantly lower efficiency than curves D and E. At the lower end of the speed range, for example between V1 and V3, curves D and E indicate significantly lower efficiency than the other curves.
In motor vehicle drives, operation efficiency is particularly important as it is desirable to extend battery life and thus the time period beyond which it becomes necessary to recharge or replace an on-board battery. The need thus exists for motors that can operate with more uniformly high efficiency over a wider speed range than those presently in use.
The present invention fulfills the above-described needs of the prior art and provides additional advantages for configurations such as the isolated individual stator core arrangements disclosed in the above identified Maslov et al. applications. Advantages are obtained, at least in part, by changing motor topology on a dynamic basis to obtain maximum efficiency for each of a plurality of operating speed ranges. A plurality of mutually exclusive speed ranges between startup and a maximum speed at which a motor can be expected to operate are identified and a different number of the motor stator winding coils that are to be energized are designated for each speed range. The number of energized coils are changed dynamically when the speed crosses a threshold between adjacent speed ranges.
Although the concepts of the present invention are applicable to a motor of any stator configuration, including a single unitary core structure, the motor illustrated for purposes of explanation has a plurality of salient pole stator core segments, each core segment corresponding to a motor phase having a winding comprising a plurality of sets of coils. Each core segment winding may be comprised of a plurality of individually separate coil sets or of a single winding with coil sets separated by tap connections. Preferably, a plurality of stator core segments are ferromagnetically isolated from each other, each core formed by a pole pair, and the coil sets of each winding are serially connected to each other at respective taps, each respective tap corresponding to a respective portion of the speed range.
At starting, all coils of each core segment winding can be energized to start motor rotation. As speed accelerates, a set of coils of each phase winding is de-energized as a speed range threshold is exceeded, while maintaining energization of the remaining coils of each core segment winding. Another set of coils is de-energized for each succeeding speed range. If the motor decelerates below a speed range threshold, a de-energized coil set is re-energized. The motor energizing source may be a switch controlled direct current source that produces desired motor current waveform profiles in accordance with signals generated by a controller. An electronic switch may be connected between each tap and one terminal of the energizing source. In response to sensed speed, a controller generates switch activation signals to be applied to the appropriate switch of each phase winding to supply energizing current to the corresponding tap. A single controller can be provided for both of these functions.
Each core segment may correspond to one phase of a multiphase motor wherein each core segment winding has the same total number of turns. Preferably, in such configuration the number of taps is the same for each core segment winding and the number of coil turns between taps is the same for each core segment winding. Each pole of a segment pole pair may have a winding formed thereon, or the coils of each winding may be distributed on both poles of the respective pole pair, or the winding may be formed on a portion linking both poles of a pole pair.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.